The Sado estuary, where the river discharge is several orders of magnitude smaller that the flow associated to the tidal prism and where the residence time is longer then 1 month. The estuary has strong tidal velocities that associated to the curvature of the estuary generating strong residual eddies with important secondary flows associated. This circulation is responsible for the formation of large sand banks inside the estuary and for strong mixing between different zones of the estuary. Although the residence time inside the estuary is long, the residence time of the water in each part of the estuary is short.

The Sado estuary is a very interesting estuary in terms of trophic activity. The discharge of nutrients in the estuary is small and the residence time is much longer than the time required for developing a bloom. As a consequence, the estuary has the behaviour of a closed basin, where the primary production is limited by nutrients supply, and predation by zooplankton. The results show the formation of typical spring blooms of phytoplankton which amplitude are limited by nutrients and by grazing. After the shooting of the phytoplankton bloom, the zooplankton also decreases and nutrients increase again in the estuary due to the river discharge and regeneration from mineralization of organic matter especially in the shallow intertidal areas. Then a new and shorter bloom develops by the end of spring, followed by another much smaller in summer. The scenario of reduction of nutrients does not change this evolution pattern, but reduces the amplitude of the blooms, especially the zooplankton bloom.